The present invention relates to the use of a fluid flow measuring device as a microphone.
Conventional microphones make use of the fact that the pressure waves associated with an acoustic signal give rise to mechanical vibrations in a membrane or the like, which vibrations are converted with the aid of suitable conversion means into an electrical varying signal, the frequencies arising in the electrical varying signal corresponding to those of the acoustic signal.
The invention is based on the fact that a pressure wave and a mass flow wave are always associated with an acoustic wave, the flow wave, however, having a phase shift relative to the pressure wave. However, the flow waves which are associated with an acoustic signal comprise the same frequency pattern as the pressure waves and are therefore also able to serve as the basis for sound measurement.
The use of a fluid flow measuring device as a microphone is known from R. O. Fehr, "Infrasonic thermistor microphone", Journal. of the Audio Engineering Society, April 1970, Vol. 18, Nr. 2, pages 128-132. This publication discloses the use of hot-wire anemometers for the measurement of turbulence. In such devices, a very fine wire is electrically heated and is cooled depending on the turbulence of the air. The temperature changes of the wire are registered as resistance variations of the wire. The resistance variations can be detected by suitable electronic measuring circuits, thus providing an electrical signal which is proportional to the volume flow. However, hot-wire microphones are insensitive to the direction of the volume flow. Moreover, hot-wire microphones double the frequencies of the volume flow variations. In the publication of R. O. Fehr, the application of two thermistors is disclosed to solve these problems of the hot-wire microphones. When there is a volume flow over one of the two thermistors, the windward thermistor will be cooled, whereas the other thermistor will receive heat transferred by the wind from the first thermistor. Again, the temperature variations of the two thermistors are translated into varying electrical signals by connecting the two thermistors in opposite arms of an electrical bridge circuit. The microphone described in this publication is reported to operate only in a frequency range from 0.1 to 20 Hz. The diameter of the thermistors used is reported to be about 330 .mu.m (13 mill). Since the publication of this article by R. O. Fehr about 25 years ago, no research has been carried out for developing a microphone based on fluid flow measurements for detecting acoustic waves in the audible range.
U.S. Pat. No. 4,932,250 suggests the use of a fluid flow measuring device as a microphone for the detection of ultrasonic waves. No arrangement is suggested to detect acoustic waves within the audible range.
From several documents a micro fluid flow measuring device is known which comprises at least one heating element and at least two temperature sensors arranged in opposite positions to the heating element; see e.g. the German patent application 36 11 614, the European patent application 0,268,004, British patent application 2,226,139, and T. S. J. Lammerink, et al., "Micro-liquid flow sensor", Sensors and Actuators A, 37-38 (1993), pages 45-50. In none of these documents reference is made to a possible use of the fluid flow sensor disclosed as a microphone.
The object of the invention is to provide a microphone which is able to detect acoustic waves in the audible frequency range based on fluid flow measurements.
In order to achieve this objective, use is preferably made of techniques and means for the measurement of fluid flow which are known per se. A typical example of a fluid flow sensor which can advantageously be used in the microphone according to the invention is described in: T. S. J. Lammerink, et al., "Micro-liquid flow sensor", Sensors and Actuators A, 37-38 (1993), 45-50, referred to above.
The invention provides the use of a fluid flow measuring device as a microphone for detecting acoustic waves, said fluid flow measuring device comprising at least one heating element, at least a first temperature sensor arranged at a first predetermined spacing from the heating element, for generation of a first electrical signal which corresponds to the temperature of the first temperature sensor wherein the predetermined first spacing is less than 300 .mu.m.
By using techniques now known from micro-electronics it is possible to construct such a fluid flow measuring device with such micro-dimensions. Surprisingly, the signal to noise ratio of such a microphone is good up to 10 kHz, or more. A good signal to noise ratio, up to 10 kHz, was observed in a practical embodiment of the invention where one heating element was accompanied by two temperature sensors located on opposite sides of the heating element at a spacing of 40 .mu.m. Good signal to noise ratios up to such high frequencies can therefore be achieved with the microphone according to the invention when the spacing between the heating element and the temperature sensors applied is less than 50 .mu.m.
A further advantage of this microphone is that it has an extremely low cut-off frequency: conventional microphones are no longer able to detect very low frequencies of an acoustic signal because of the inherent rigidity of the membrane or the like measuring the pressure wave. In a microphone according to the invention, on the other hand, very low frequencies of an acoustic signal are converted into very low frequencies of a thermal signal, which is converted without attenuation into a low-frequency electrical signal.
The invention also relates to a system comprising a microphone comprising a fluid flow measuring device for detecting acoustic waves, said fluid flow measuring device comprising at least one heating element, at least a first temperature sensor arranged at a first predetermined spacing from the heating element, for generation of a first electrical signal which corresponds to the temperature of the first temperature sensor wherein the predetermined first spacing is less than 300 .mu.m, the microphone also comprising an electronic measuring circuit for measuring said first electrical signal and providing an electrical output signal, the system further comprising an amplifier for amplifying said output signal and providing an amplified output signal, and a loudspeaker connected to said amplifier.
A microphone based on the measurement of fluid flow waves may be advantageously used in combination with a microphone based on pressure wave measurements. With an arrangement of this type, both the flow waves and the pressure waves of an acoustic signal can be measured, as a result of which the possibility exists of determining not only the magnitude but also the absolute propagation direction of the propagating acoustic signal.
Advantageous embodiments of the use of a fluid flow measuring device as a microphone, and of the system comprising a microphone based on fluid flow measurements are also defined.